Replacement of the cathode ray tube with a flat panel screen requires a display technology which simultaneously makes it possible to achieve a high image resolution, i.e. more than 1000 lines, a high image brightness ( greater than 200 cd/m2), a high contrast ( greater than 100:1), a high frame rate ( greater than 60 Hz), an adequate color representation ( greater than 16 million colors), a large image format (screen diagonal  greater than 40 cm), a low power consumption and a wide viewing angle, at low production costs. At present, there is no technology which fully satisfies all these features simultaneously.
Many manufacturers have developed screens which are based on nematic liquid crystals and have been used in recent years in the field of notebook PCs, Personal Digital Assistants, desktop monitors etc. Use is made here of the technologies STN (supertwisted nematics), AM-TN (active matrixxe2x80x94twisted nematics) AM-IPS (active matrixxe2x80x94in-plane switching) and AM-MVA (active matrixxe2x80x94multidomain vertically aligned), which are described in the relevant literature; see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein; SID Symposium 1997, ISSN-0097-966X pages 7 to 10, 15 to 18, 47 to 51, 213 to 216, 383 to 386, 397 to 404 and the references cited therein. Furthermore, use is being made of the technologies PDP (plasma display panel), PALC (plasma addressed liquid crystal), ELD (electroluminescent display), FED (field emission display) etc., which are also explained in the above-cited SID report.
Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically suitable for areas of application such as computer displays and TV sets, as shown by a monitor marketed in Japan by Canon since May 1995.
The use of FLCs in electro-optical or fully optical components requires either compounds which form smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range.
The individual pixels of an LC display are usually arranged in an x,y matrix formed by the arrangement of a series of electrodes (conductor tracks) along the rows and a series of electrodes along the columns on the upper or lower side of the display. The points of intersection of the horizontal (row) electrodes and the vertical (column) electrodes form addressable pixels.
This arrangement of the pixels is usually referred to as a passive matrix. For addressing, various multiplex schemes have been developed, as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93, and Kontakte 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler display production and consequently lower production costs, but the disadvantage that passive addressing can only be carried out line by line, which results in the addressing time for the entire screen with N lines being N times the line addressing time. For usual line addressing times of about 50 microseconds, this means a screen addressing time of about 60 milliseconds in, for example, the HDTV (high definition TV, 1152 lines) standard, i.e. a maximum frame rate of about 16 Hz, too slow for moving images. In addition, display of gray shades is difficult. At the FLC Conference in Brest, France (Jul. 20-24, 1997, see Abstract Book 6th International Conference on Ferroelectric Liquid Crystals, Brest/France), a passive FLC display with digital gray shades was shown by Mizutani et al., in which each of the RGB pixels (RGB=red, green, blue) was divided into sub-pixels, allowing the display of gray shades in digital form through partial switching. Using three basic colors (red, green, blue), N gray shades result in 3N colors. The disadvantage of this method is the considerable increase in the number of screen drivers necessary and thus in the costs. In the case of the display shown in Brest, three times the number of drivers were necessary than in a standard FLC display without digital gray shades.
In so-called active matrix technology (AMLCD), a nonstructured substrate is usually combined with an active matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated into each pixel of the active matrix substrate. The nonlinear elements can also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the relevant literature; see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein.
Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in alignment and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of these processes is the poor video capability owing to excessively slow response times of nematic liquid crystals.
For this and other reasons, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, see for example WO 97/12355, or Ferroelectrics 1996, 179, 141-152, W. J. A. M. Hartmann, IEEE Trans. Electron. Devices 1989, 36 (9; Pt. 1), 1895-9.
Hartmann utilized a combination of the so-called xe2x80x9cquasi-bookshelf geometryxe2x80x9d (QBG) of an FLC and a TFT (thin-film transistor) active matrix to simultaneously achieve high response speed, gray shades and high transmission. However, the QBG is not stable over a broad temperature range, since the temperature dependence of the smectic layer thickness disrupts or rotates the field-induced layer structure. Moreover, Hartmann utilizes an FLC material having a spontaneous polarization of more than 20 nC/cm2, which, for pixels having realistic dimensions of, for example, an area of 0.01 mm2, leads to high electric charges (at saturation, Q=2 A P, A=pixel area, P=spontaneous polarization). With low-cost amorphous silicium TFTs, for example, these high charges cannot reach the pixel in the course of the opening time of the TFT. For these reasons, this technology has not been pursued any further to date.
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have suggested a monostable FLC geometry (see Journal of the SID, 1/2, 1993, pages 163-169) in which the FLC material is aligned by means of relatively high voltages such that only a single stable position results from which a number of intermediate states are generated by application of an electric field via a thin-film transistor. These intermediate states correspond to a number of different brightness values (gray shades) when the cell geometry is matched between crossed polarizers.
One disadvantage of this technique is the occurrence of a streaky texture in the display which limits contrast and brightness of this cell (see FIG. 8 in the abovementioned citation). While it is possible to correct the disadvantageous streaky texture by treatment with a high electric voltage (20-50 V) in the nematic or cholesteric phase (see page 168 of the abovementioned citation), such a field treatment is unsuitable for mass production of screens and usually does not result in temperature-stable textures. Furthermore, this method produces switching only in an angle range of up to a maximum of once the tilt angle, which is about 22xc2x0 in the case of the material used by Nito et al. (cf. p. 165, FIG. 6) and thus produces a maximum transmission of only 50% of the transmission of two parallel polarizers.
The object of the present invention is to provide a ferroelectric active matrix liquid crystal display comprising a ferroelectric liquid crystal mixture, where the liquid crystal mixture assumes a monostable position, but without forming a streaky texture, is temperature-stable and makes it possible to achieve a very high maximum transmission and a very high contrast.
This object is achieved according to the invention by a monostable ferroelectric active matrix display comprising a liquid-crystal layer, preferably in chevron C2 geometry, in the form of a monodomain having an unambiguously defined direction of the layer normal z of the chiral smectic phase, the ratio between the sum of pretilt angle and layer leaning angle and tilt angle (AR=(LLA+PTA)/TIA) being greater than 0.1 and the absolute value of the dielectric anisotropy DA being preferably less than 3.
The spontaneous polarization P in the liquid-crystal layer is preferably between 0.1 and 15 nC/cm2.
The tilt angle TIA in the liquid-crystal layer is preferably between 9 and 40xc2x0.
The ratio between the product of anchoring strength and the sinus of the tilt angle and the spontaneous polarization (AS sin TIA/P) is preferably less than 20 V/xcexcm.
The object is furthermore achieved by a monostable ferroelectric active matrix display comprising a liquid-crystal layer in chevron C1 geometry in the form of a monodomain having an unambiguously defined direction of the layer normal z of the smC* phase, the pretilt angle PTA being at least 5xc2x0 and the ratio between pretilt angle and layer leaning angle (PTA/LLA) being greater than 0.7.
The object is furthermore achieved by a monostable ferroelectric active matrix display comprising a liquid-crystal layer in bookshelf geometry in the form of a monodomain having an unambiguously defined direction of the layer normal z of the smC* phase, the pretilt angle PTA being at least 1xc2x0.
The object is furthermore achieved by a monostable ferroelectric active matrix display comprising a liquid-crystal layer in the form of a monodomain having an unambiguously defined direction of the layer normal z of the smC* phase having the following properties:
a spontaneous polarization P of between 0.1 and 15 nC/cm2,
a tilt of 9xc2x0 to 45xc2x0 C.
an absolute value of the ratio between layer leaning angle and tilt angle of at least 0.2,
a pitch in the chiral nematic (cholesteric) phase of at least 50 xcexcm within the temperature range of 5xc2x0 C. above the smectic-nematic phase transition or, if the range of existence of the nematic (cholesteric) phase is less than 5xc2x0 C., within a temperature range of at least 80% of the nematic phase range, and
an absolute value of the dielectric anisotropy DA of less than 3.
The liquid-crystal layer preferably has one or more, in particular all, of the following features:
the angle between the layer normal z of the smC* phase and the preferential direction n of the nematic or cholesteric phase (N* phase) is in the range from 0.5 to 1.0 times the smC* tilt angle, but at least 5xc2x0,
the ferroelectric liquid-crystal layer has the phase sequence
I*xe2x88x92N*xe2x88x92smC*
where an smA* phase having a range of existence of not more than 20xc2x0, preferably not more than 1xc2x0, may exist between the N* phase and the smC* phase.
The active-matrix FLCD of the invention preferably comprises, as optically active layer, a ferroelectric liquid-crystalline medium (liquid-crystal phase) having a phase sequence of
isotropicxe2x88x92nematic or cholesteric (N*)xe2x88x92smectic C*
or a phase sequence of
isotropicxe2x88x92nematic or cholesteric (N*)xe2x88x92smectic A*xe2x88x92smectic C*,
where the smectic A* phase has a range of existence (please range) of not more than 2xc2x0 C., preferably not more than 1xc2x0 C., particularly preferably not more than 0.5xc2x0 C. The asterisk (*) attached to the phase name indicates a chiral phase.
The displays are preferably produced by a process, which comprises introducing the liquid-crystal layer into the space between a rubbed upper substrate plate and a rubbed lower substrate plate of the active matrix display, the rubbing directions on the upper substrate plate and the lower substrate plate being essentially parallel, and cooling the liquid crystal phase from the isotropic phase, an electric voltage being applied to the display at least during the N*xe2x86x92smC* or N*xe2x86x92smA*xe2x86x92smC* phase transition.
The FLC mixture is filled into an active matrix display. Production and components of an AM display of this type are described in the above-cited Tsukuda reference. However, in contrast to nematic displays, the thickness of the FLC layer is only from 0.7 to 2,5 xcexcm, preferably 1-2 xcexcm. Moreover, the rubbing directions on upper and lower substrate plates are essentially parallel. The term xe2x80x9cessentially parallelxe2x80x9d includes antiparallel rubbing directions or rubbing directions which are weakly crossed, i.e. up to 10%.
It is important for the operation of this display that in the production of the display, during controlled cooling, a direct electric current, preferably of less than 5V, is applied and maintained during the N*xe2x86x92smC* or N*xe2x86x92smA*xe2x86x92smC* phase transition, with the result that the whole display assumes a monostable monodomain which appears completely dark between crossed polarizers.
After this domain has been obtained, the direct current is switched off. In contrast to the abovementioned approach by Hartmann or conventional bistable FLCDs, the resulting texture is monostable. This means that the preferred n director (which indicates the preferential direction of the long axes of the molecules) is in the rubbing direction of the cell, whereas the z director (which indicates the preferential direction of the smectic layer normal) is oblique relative to the rubbing direction by approximately the tilt angle value. This constellation is exactly the opposite of the conventional bistable cell according to Clark and Lagerwall in which the z director is in the rubbing direction.
In contrast to Nito""s approach, this is exactly the orientation in which there are no two layer normals and no two orientation domains, which ultimately lead to the unwanted streaky texture described above, but a single unambiguous direction of the z director and thus a single monodomain only. Furthermore, it is possible to obtain twice the tilt angle, which leads to 100% transmission, based on parallel polarizers, i.e. double brightness is achieved.
At a suitable angle of rotation, the resulting display appears completely dark between crossed polarizers. On applying an addressing voltage of only a few volts, the display appears bright, it being possible to vary the brightness continuously by means of the voltage, and is almost as bright as two parallel polarizing films when saturated. The angle between the preferential direction of the nematic (or cholesteric) phase and the layer normal (z director) is ideally and thus preferably equal to the tilt angle of the smectic C phase, or at least essentially equal to the tilt angle. For the purposes of the invention, xe2x80x9cessentiallyxe2x80x9d means preferably a range from half the tilt angle to the full tilt angle, particularly preferably from 0.5 to 1.0 times the tilt angle, but at least 5xc2x0.
The ferroelectric active matrix liquid crystal display of the invention is particularly useful in practice, in particular for TV, HDTV or multimedia, since it combines high transmission, short response times, gray scale and thus full color capability, low-cost production and a broad temperature range. Furthermore, the display can be operated at voltages of xe2x89xa610 volts, preferably of xe2x89xa68 V, particularly preferably of xe2x89xa65 V.
For display of gray shades or as many natural colors as possible, the characteristic line (transmission plotted against voltage) of the liquid crystal mixture should be sufficiently flat to address the gray shades reliably using the available voltages, and the saturation voltage should not be too high.
The saturation voltage V90, at which 90% of maximum transmission are achieved, should not be too high so as to allow operation of the display at below 30 V, preferably below 15 V, more preferably below 10 V, particularly preferably below 8 V, especially below 5 V. The threshold voltage V10 should preferably be adapted to V90 so as to ensure that the the characteristic line width CLW is large enough to be able to address a sufficiently high number of gray shades. This is generally the case if the characteristic line width CLW=V90xe2x88x92V10 is at least 100 mV, preferably at least 200 mV, more preferably at least 500 mV, particularly preferably at least 1 V, especially at least 1.5 V.
Furthermore, the maximum transmission of the cell should be at least 40% (based on an empty cell between two parallel polarizing films), preferably at least 50%, more preferably at least 70%, particularly preferably at least 80%, especially at least 90%. Moreover, the T,V characteristic line should preferably increase strictly monotonically (with increasing voltage). A fall in transmission after reaching a transmission maximum is undesirable.
The invention accordingly provides the selection of liquid crystals and mixtures thereof having suitable material parameters for advantageous adjustment of the characteristic line.
The invention furthermore provides a monostable active matrix FLC display, in which an optimum characteristic line is achieved by selection of liquid crystals or mixtures thereof by specific combination of a plurality of material properties of the ferroelectric liquid crystal, and the use of liquid crystal mixtures having these properties for active matrix FLC displays.
In particular, the term xe2x80x9cactive matrix displayxe2x80x9d as used herein includes an LCD in which one of the two substrates is replaced by the rear side of an IC chip (IC=integrated circuit) as described, for example, in D. M. Walba, Science 270, 250-251 (1995).
The abbreviations used in the examples and in the description of the invention are explained in the table below.
The C1, C2 geometries etc. are described by D. C. Ulrich and S. J. Elston in Ferroelectrics, vol. 178, p. 177-186 (1996).
The characteristic line of the ferroelectric liquid crystal cell of the invention is influenced by a number of parameters which, alone or in combination, should be in preferred ranges so that optimum switiching conditions are achieved. These parameters are in particular the spontaneous polarization (P), the tilt angle (TIA), the layer leaning angle (LLA), the pretilt angle (PTA), the anchoring strength (AS), the dielectric anisotropy (DA), furthermore the layer rotation angle (LRA), the cell thickness (d), the pitch of the cholesteric phase and the smectic C* phase and the optical anisotropy (OA).
It is found that all these parameters influence the characteristic line, although to a varying extent. The characteristic line (T,V) should preferably have the following characteristics.
In contrast to all usual experiences made with ferroelectric LCDs, it has been found that the characteristic line is not influenced, or influenced only to a small extent, by the rotational viscosity; instead, e.g. the saturation voltage V90 is strongly dependent on the spontaneous polarization (P) and the anchoring strength (AS).
The spontaneous polarization (P) should preferably be between 0.1 and 15 nC/cm2 (here, this always means the absolute value of P), preferably between 0.2 and 10 nC/cm2, more preferably between 0.4 and 8 nC/cm2, particularly preferably between 0.5 and 6 nC/cm2, especially between 0.8 and 3.5 nC/cm2.
The tilt angle should preferably be in the range from 9xc2x0 to 45xc2x0, preferably between 12xc2x0 and 35xc2x0, more preferably between 14xc2x0 and 31xc2x0, particularly preferably between 17xc2x0 and 27xc2x0, especially between 19xc2x0 and 25xc2x0.
The layer rotation angle (LRA=angle between the preferential direction of the nematic phase and the smC* layer normal) should preferably be at least 5xc2x0.
The absolute value of the dielectric anisotropy should preferably be less than 3 (three), more preferably less than 2.5, particularly preferably less than 1.8, especially less than 1.2.
The product of anchoring strength (AS) and the sinus of the tilt angle divided by the spontaneous polarization (P) should preferably be less than 20 V/xcexcm, preferably less than 15 V/xcexcm, more preferably less than 12 V/xcexcm, particularly preferably less than 9 V/xcexcm, especially less than 6 V/xcexcm.
The parameter indications relate to at least one temperature in the operating range of the ferroelectric liquid crystal display.
The display of the invention can be operated not only in the range of the smectic C* phase, but alsoxe2x80x94at least partiallyxe2x80x94in the range of another tilted smectic phase, with the abovementioned properties being applied by analogy.
The monostable active matrix FLC display of the invention can be operated in chevron C1 geometry, chevron C2 geometry or bookshelf or quasi-bookshelf geometry, respectively. For all three geometries, the preferred combination of ranges of spontaneous polarization and tilt angle values is valid:
Even more preferred are abovementioned combinations of P and TIA together with an absolute value of the dielectric anisotropy which is less than 3 (three), more preferably less than 2 (two), particularly preferably less than 1.5, especially less than 1.2.
The pitch of the cholesteric helix should be at least 50 xcexcm within the temperature range of 5xc2x0 above the smectic phase transition or, if the range of existence of the cholesteric phase is smaller, preferably within a temperature range of 80% of this range of existence. It is preferred to achieve a pitch of at least 70 xcexcm, particularly preferably of at least 100 xcexcm, to achieve high contrasts.
In the C2 geometry, which is usually preferred at small pretilt angles and completely disappears at large pretilt angles in the limiting case (PTA greater than TIA), the layer leaning angle (LLA) together with the pretilt angle (PTA) should preferably relate to the tilt angle (TIA) as follows: the ratio AR between the sum of pretilt angle and layer leaning angle and the tilt angle (i.e. (LLA+PTA)/TIA) should generally be at least 0.1, preferably at least 0.15, more preferably at least 0.25, particularly preferably at least 0.5, especially at least 0.7. Here, the absolute value of the dielectric anisotropy DA should be less than 3.
Most particularly, advantageous characteristic lines are achieved using the combinations listed in the table below:
Even more preferred are abovementioned combinations of P and TIA together with an absolute value of the dielectric anisotropy which is less than 3, more preferably less than 2, particularly preferably less than 1.5, especially less than 1.2.
The C2 geometry is generally preferred over C1 and bookshelf.
In the C1 geometry, which is preferred at large pretilt angles, the pretilt angle should be at least 5xc2x0, at a suitable layer leaning angle, since otherwise no switching occurs at low voltage.
At a suitable layer leaning angle, the relation between layer leaning angle (LLA) and pretilt angle (PTA) should advantageously be as follows: the ratio between pretilt angle and layer leaning angle should be greater than 0.7.
Most particularly, advantageous characteristic lines are achieved using the combinations listed in the table below (for C1 geometry):
Even more preferred are abovementioned combinations of P and TIA together with a dielectric anisotropy which is greater than xe2x88x921, preferably with a positive dielectric anisotropy, i.e.  greater than 0.
In the bookshelf geometry, which is herein defined for the range xe2x88x925xc2x0 less than LLA greater than +5xc2x0 (ideally LLA=0), the pretilt angle should generally be at least 1xc2x0, preferably at least 2xc2x0. Particulary preferably, the pretilt angle should be at least 1xc2x0, the spontaneous polarization should be at least 0.1 nC/cm2 and not more than 15 nC/cm2, and the tilt angle should be at least 12xc2x0.
In the tables above, the spontaneous polarization can also preferably be from 0.1 to 15 nC/cm2.
A mixture which is particularly suitable for use in the display of the invention comprises at least six (6), preferably at least eight (8), particularly preferably at least nine (9), especially at least eleven (11), components which are selected such that the spontaneous polarization is between 0.1 and 15 nC/cm2, the tilt angle is between 17 and 27xc2x0, the ratio between layer leaning angle and tilt angle is at least 0.3, the pitch of the chiral nematic phase is at least 50 xcexcm (range of 5xc2x0 C. above the phase transition), the absolute value of the dielectric anisotropy is less than 3, and the phase sequence is
xe2x80x83isotropicxe2x88x92nematicxe2x88x92smectic C*
or
isotropicxe2x88x92nematicxe2x88x92smectic Axe2x88x92smectic C*,
where the smectic A phase has a range of existence of not more than 2xc2x0 C.